This application is the national phase under 35 U.S.C. xc2xa7371 of PCT International Application No. PCT/JP98/03592 which has an International filing date of Aug. 12, 1998, which designated the United States of America.
The present invention relates to a polytetrafluoroethylene tube which has a small wall thickness deviation and an apparatus and a process for the production of such a tube.
As a production process of a polytetrafluoroethylene tube, a paste extrusion process has been hitherto known. In that process, a paste material is prepared by adding a suitable organic solvent called as a lubricant into a fine powder of a polytetrafluoroethylene (hereinafter, referred so as also xe2x80x9cPTFExe2x80x9d) produced by emulsion polymerization; the paste material is preformed into a cylinder so as to obtain a billet; the billet is charged into an extruder which extrudes the billet into a tube form while pressing using a ram; and then the lubricant is removed from the tube by heating or extraction followed by sintering the tube so as to finally obtain a predetermined tube (see, for example, Fluoro-Resin Handbook edited by Takaomi Satokawa published by Nikkan-Kogyo-Shinbun-sha (1990)).
A cross-section of an end portion a ram extruder used for the above process is schematically shown in FIG. 1 from which portion a tube is extruded. The extruder 10 comprises a cylinder 12, a master die 14 and a die orifice 16, which form a space 18. A mandrel 20 and a core pin 22 which is attached to the mandrel are coaxially located within the space 18. A billet is supplied into such a space of the extruder. When the billet is extruded by a ram (not shown) from the upper side to the lower side of the figure, a tube is obtained from an annular outlet 24 of the die orifice. A lubricant is removed from the tube, which is then sintered so as to finally produce a predetermined tube.
When a shaped article is produced in the form of a tube from the polytetrafluoroethylene in the above described process, the mandrel 20 and the core pin 22 have to be so located that their axes are aligned with a center axis of the space 18 which is formed by the cylinder 12, the master die 14 and the die orifice 16. However, one end of the core pin 22 cannot be supported at a level of an outlet 24 of the die orifice 16, so that the core pin is cantilevered at a lower end of the mandrel 20 to which the core pin is attached, and such mandrel is cantilevered at its upper end. Therefore, it is not easy at all to a center the core pin 22 with the die orifice 16 around at the level near the outlet 24 of the die orifice 16. In fact, it is extremely difficult and almost impossible to substantially align a center axis of the core pin 22 with a center axis of the die orifice 16. As a result, a wall thickness of a currently and commercially available PTFE tube, especially one which has a lager diameter (for example, a tube having an inner diameter of not smaller than 10 mm), fluctuates largely. That is, wall thickness deviation phenomenon occurs in that a thickness in a radial direction (wall thickness) of the tube across its cross-section which is perpendicular to a longitudinal direction of the tube is different depending on a radial orientation.
Therefore, it is an object of the present invention to minimize the wall thickness deviation of the tubular article produced by paste extrusion of the PTFE.
The present inventors have made extensive studies so as to achieve the above object, and found that dimensioning a core pin such that at least a portion thereof can bend (or deflect) due to a pressure applied upon the extrusion provides an automatic centering function with an extruder (namely, a function to automatically align an axis of a core pin with an axis of a die orifice), so that the above object is achieved.
Generally, an elongated object which is supported at a single point is placed in a fluid which is flowing, the object moves in such a direction that a resistance which acts on the object is likely to be minimized. That is, when a supported portion of the object is able to freely rotate, the elongated object so moves that its longitudinal direction is to be parallel to a flowing direction of the fluid; and when the supported portion is constrained, the object itself bends such that the resistance is minimized. The present inventors have found that when a phenomenon similar to the above phenomenon occurs in the paste extruder for the PTFE, the core pin is so located that resistance acting on the core pin is minimized, so that the core pin is centered with the die orifice, whereby a polytetrafluroethylene tube having a minimized wall thickness deviation is obtained.
Generally, in an extruder, a mandrel is mechanically fixed at its one end which is opposed to a die orifice outlet while aligned with a cylinder, and a core pin is fixed to the mandrel at the other end thereof. The reason why a tube extruded while pressing in a longitudinal direction of the mandrel by a ram using such extruder has the wall thickness deviation is that the core pin is decentered because of a certain cause even if the mandrel is firmly fixed (namely, the core pin becomes not concentric with the die orifice, and for example, a center axis of the core pin is inclined relative to a center axis of the die orifice), and such decentered state is enlarged on an outlet side of the die orifice so that the center of the core pin is greatly deviated from the center of the die orifice. Upon such deviation, on a side where a thinner wall thickness of the tube is formed, the core pin forms an angle which is smaller than 180xc2x0 with respect to a direction of an overall resin flow in a cylinder caused by ram pressing, namely a direction of a center axis of the cylinder (namely, the angle being formed between the mandrel and the core pin). To the contrary, on a sidle where a thicker wall thickness of the tube is formed, the core pin forms an angle which is larger than 180xc2x0 with respect to the direction of the overall resin flow (namely, the angle being formed between the mandrel and the core pin).
In the above situation, the angle which is smaller than 180xc2x0 produces a resistance against the resin flow, the core pin tries to move so as to reduce the resistance (namely, the core pin tries to approach the center axis of the die orifice). If the core pin could move so as to reduce the resistance, as a result of this the center of the core pin can substantially approach the center of the die orifice at a level of the outlet end of the die orifice. That is, the core pin is able to move so as to minimize the wall thickness deviation.
For example, in the case where the center of the core pin is deviated from the center of the die orifice at a level near the die orifice outlet due to a certain reason before the extrusion, whether the center of the core pin can or cannot return to the center of the die orifice (namely, becoming a concentric state) or approach the center of the die orifice at the level near the die orifice outlet depends on whether the core pin can or cannot bend (or deflect) under the extrusion conditions. Thus, the present invention resides in that shaping the core pin of the extruder such that it bends more easily than the conventional (or prior art) core pin so that the center of the core pin easily returns to the center of the die orifice around the core pin, which leads to the improved centering condition, whereby the wall thickness deviation of the extruded tube is improved.
Thus, in the first aspect, the present invention provides an extruder which produces a polytetrafluoroethylene tube by extruding a polytetrafluoroethylene through an annular portion which is formed by a core pin and a die orifice around the core pin, characterized in that the core pin comprises a mandrel connection end which is connected to a mandrel and a die orifice outlet end which is located at a level of a die orifice outlet, and the core pin has between its both ends a portion of which cross-section perpendicular to an axial direction of the core pin has a smaller area than that of a cross-section of each end of the core pin.
In the conventional extruder, the core pin was not designed while considering the bend of the core pin as described above. The core pin has a diameter at its one end which is substantially the same as a diameter of an end of the mandrel of the extruder so that the core pin is connected to the mandrel end, and also has a diameter at the other end which substantially corresponds to an inner diameter of a tube to be extruded. A diameter of a portion between those ends is gradually changed from one diameter to the other diameter (namely, an area of the cross-section of the core pin which is perpendicular to an axis direction of the core pin is gradually increase or decrease, or remains constant). It is noted that the core pin includes a portion of which diameter is constant, and not changed as in a core pin of a comparative example which will be described later, but the diameter of such a portion does not become smaller than a diameter of a cross-section of the mandrel or a cross-section of the core pin at a level of the die orifice outlet. Therefore, a diameter of a portion of the conventional core pin between its both ends is equal to or greater than a diameter at one end and equal to or smaller than a diameter at the other end, or such diameter is equal to a diameter at one end and also equal to a diameter at the other end.
To the contrary, in the extruder according to the present invention, the core pin has the portion of which cross-section is smaller than its both ends, so that the core pin more easily bend than that of the conventional extruder which does not have such a portion.